1. Field of the Invention
The present invention relates to an optical multimode interference device (hereinafter called an “MMI” device), and a manufacturing method for the same, to be used in optical communication, optical switching, optical wiring, and the like.
2. Description of the Related Art
FIG. 2A and FIG. 2B shows a plan view illustrating the structure of a conventional MMI device. Literature that discloses the conventional art includes Japanese Patent Laid-Open Publication No. 2001-215452, and U.S. Pat. No. 5,563,968.
With the conventional MMI device as shown in FIG. 2A, for example, narrow width single-mode waveguides A, B are provided at one end in a longitudinal direction of a rectangular parallelepiped forming a waveguide for light, and at the other end, narrow width single-mode waveguides C, D are provided. Between the single-mode waveguides A, B and the single-mode waveguides C, D, a broad width multimode waveguide M is provided. In addition, between the single-mode waveguides A, B, an end face T1 is formed perpendicularly to the single-mode waveguides A, B. In addition, between the single-mode waveguides C, D, an end face T2 is formed perpendicularly to the single-mode waveguide C, D.
With such an MMI device, light which is introduced into the single-mode waveguide A is coupled into a plurality of multimodes in the broad width multimode waveguide M, and is emitted into the single-mode waveguides C, D on the light emission side, being demultiplexed by the interference effect between the multimodes. Thereby, the MMI device performs the function of a demultiplexer. In addition, it also functions as a multiplexer with which, when different light beams are introduced into the single-mode waveguides A, B, respectively, these light beams are multiplexed to be outputted from the single-mode waveguides C, D.
However, it is known, with the MMI device as shown in FIG. 2A, that the light which is introduced into the single-mode waveguide A and is passed through the multimode waveguide M is incident on the perpendicular end face T2 at the light emission end, and a part of the light is reflected to be returned to the single-mode waveguide A, resulting in the characteristics of the light source connected to this single-mode waveguide A being adversely influenced.
On the other hand, with the MMI device as shown in FIG. 2B, which is described in U.S. Pat. No. 5,563,968, the end face T1 between the single-mode waveguides A, B, and the end face T2 between the single-mode waveguides C, D are inclined with respect to the optical axis within the waveguide, whereby the light which is incident on the end face T2 is reflected at an angle in the plane of the waveguide, resulting in the reflected light being prevented from directly returning to the single-mode waveguide A.
However, with the MMI device as shown in FIG. 2B, although the influence of the reflection is reduced as compared to the MMI device as shown in FIG. 2A, the light reflected at the end face T2 on the light emission side is again reflected at the end face T1 on the incident side to be returned to the end face T2 on the light emission side, and such a reflection is repeated in the plane of the waveguide. Further, a part of the reflected light is returned to the single-mode waveguide A. Therefore, there has been presented a problem that, while compared to the MMI device as shown in FIG. 2A the influence of the reflection is reduced, the influence of the reflection cannot be completely eliminated.